(639c) Charge of Reactive Hydrogen at Fluid-Transition Metal or Fluid-Transition Metal Sulfide Interfaces and Their Catalytic Roles in Hydrogenation and Hydrotreating Catalysis

Activation of hydrogen on transition metal or transition metal sulfide catalysts is a ubiquitous catalytic step found within hydrotreating, hydrodeoxygenation, and hydrogenation catalysis. The hydrogen activation step, appeared at first glance to be straight forward, has been described as either homolytic cleavage on transition metal surfaces or heterolytic cleavage on metal-oxide surfaces. The resulting reactive hydrogen species carry different electronic charges, depending on the chemical property of the ligand (e.g., sulfur or oxygen anions) to which the hydrogen is associated with and on that of the solvent environment. Herein, we will describe the common mechanistic traits across the reduction reactions, in which the hydrogen dissociates and then participates in reduction of C=C and C=X (X = N, O, S) bonds, in disruption of the aromaticity in arenes, as well as in cleaving the strong C-O bonds. Specific focuses are the formation of interfacial protons, through coordination with sulfur anions and through coordination with hydrogen bond network afforded by the protic solvent and their catalytic roles in the attack of carbonyls, arenes and, for methoxy substituted arenes, the sequential cleavage of the strong Ar-OCH₃ bonds. The emphasis is on the general mechanistic traits and requirements across these reduction reactions and how the charges of reactive hydrogen may mediate the overall free energy landscape of the reactions. Understanding the electronic charge of the hydrogen would allow us to manipulate the charge through the design of local reaction environment and thus promote catalytic rates and selectivity in hydrogenation and hydrodeoxygenation catalysis.